Not my day - the wretched laptop just trashed my second attempt at replying. Apologies if this is a bit terse, but it's the third time I've tried to type this!
OK, firstly apologies for the mess. There is actually a warning about this at the end of the guide, but I'll have to make it more prominent
So, options:
1) Just in case, did you take a backup from recovery before formatting things? If so, restore that backup now and you are back where you were when you made the backup.
If not, I'm going to assume that you only formatted system, i.e. have not formatted /data (or done a factory reset). If that's not the case and you wiped data as well, then just choose a custom ROM (below) and install it, as you no longer have anything to lose.
Assuming that you have not erased data, go into recovery and take a backup now. You've got no ROM, so you can't just restore the entire backup, but it means you will have your data backed-up, which gives us more options for recovering them.
There are then 2 approaches you can take:
2) Install a custom ROM.
The All Things Root Guide sticky post has a link to a
list of ROMs at XDA-developers. You can choose a ROM from that list, copy it to your SD card (either using a card reader, or you can mount the card over usb from inside recovery), then install the zip using recovery. This will give you a new system.
The catch is that system data are generally incompatible between different ROMs, so you usually do a factory reset when changing ROM. We don't want to do that, because we're trying to save your data. So the trick is to choose a ROM that's sufficiently close to your original one that the data are compatible and we don't need to wipe. So what you are looking for is a "rooted stock ROM" that's based on as close as possible to the android version you were running before. Then you install without wiping, and with any luck it will boot. Then you can back everything up properly!
To recommend a ROM I'd need to know what your previous software was - at least whether it was 2.2 or 2.3, ideally more precisely if you know (that only applies for 2.2 - there is only one official 2.3 for the Desire). The biggest problem is that these are so old now that many of the download links are now gone, but I still have copies of a couple I could get to you via PM and dropbox. Please just tell us whatever you can about the software you were running (including when it was last updated) and we'll see what we can do.
If we can't get one which boots without a wipe, the other option is to wipe anyway, then buy the paid version of Titanium Backup. This can retrieve data and apps from a recovery backup, which is why I said to take one of those before we start. It should get your user apps back. If contacts are saved with Google they are safe. Messages are trickiest - you can try restoring these (and, if necessary, contacts) via Titanium, but if the ROM is not compatible it may not work. This is another reason for picking as close a ROM to the previous one as possible.
3) There's one other option, which is a ROM Update Utility (RUU). This will erase the phone an install a complete set of official software. It will also overwrite your recovery, so you'd need to reinstall that afterwards. But you will still be S-Off, so you won't need to run revolutionary again.
That will get the phone going, but it will erase your data. This is where the backup comes in: if you have a recovery backup of your data you can run the RUU, reinstall 4Ext, then restore the data from the backup. Provided the android version in the RUU is close enough to the original software this should work. Personally I'd be tempted to try the rooted stock ROM approach first, but this gives another option if that doesn't work for any reason.
We get RUUs from
shipped-ROMs - that link is for GSM handsets (which I'm assuming you have. Please say if not, though we have many fewer options for CDMA). Since you've run Revolutionary you should be able to use most of those, but what you want is the RUU (not OTA) that's closest to your previous software. You run the RUU by putting the phone in fastboot mode, connecting it to a PC, then running the RUU .exe on a PC (the 2.3 RUU is inside a zip, so unzip to get the .exe).
Hopefully one of those methods will let you not only get the phone working again (which is the easy bit, though it may not feel it right now!) but also keep your data intact.
So as I say, if you tell us what you were running previously we can provide some more specific suggestions.
(P.S. You are right - I don't know the meaning of the word "terse"
)