Hi Daisy,
Power it off, then restart by holding POWER+MENU to boot into recovery. Wipe the cache, then go to Advanced and wipe the dalvik-cache too. Then go back to the main menu and select Reboot system.... hopefully that will fix the problem.
If not, boot back into recovery and in Advanced select Fix Permissions. Let it complete (may take a few minutes so be patient) then restart.... hopefully this will.... etc
If still no luck, try the first option again but this time instead of restarting after wiping the cache partitions reflash CM7.1. As you're not wiping the data partition your settings and apps will remain but the system files will be refreshed. That should definitely fix it!