what ROM did you install (if any?)? did you overclock the CPU/GPU? was the device hot? usually Android devices don't phantom reboot and lose data unless something went wrong or got overclocked. this can happen with badly written custom ROMs, strange easier-than-it-should-be rooting tools hiding nasty surprises, etc. when it reset did it reboot or self-reset? if a phone it may have had a carrier-firmware installer that saw the root and basically factory reset it to 'unroot' it. there are apps that prevent this, which is why one must really do their homework before attempting a root on any device.
the best advice when rooting any phone/tablet/mp3 Player running Android is to install a custom recovery (aka clockworkmod) and make a BACKUP ALWAYS. this way if something happens later on you can restore your apps/data. it is also a good idea to backup before rooting
EDIT: the brand name 'maylong' sounds like a knock-off cheapie. sad to say many of these things fail out-of-box, and rooting can only accelerate their failure rate