While I was playing a game my computer froze up so I did a restart and it said one of my hdd failed in raid0. So I deleted the raid and put the good hdd back in my computer then the other one and made a new raid0 and reinstalled win7 now it is perfectly fine. Should I keep using the failed hdd or go get a new one? also is this common for a hdd to fail then work again?
HDD's can drop out of a RAID array for reasons other than failure -- if you had to reset the computer for a freeze, the drives might've gotten out of synch (e.g. the RAID controller finished a write on one drive but not the other, so on reboot the controller saw that, "hey, the other drive isn't showing that write I did -- it's not safe to use, so I'll kick it out of the array as a precaution").
Check the SMART log on the drive that failed -- if it's completely clean (no errors logged), it means that very likely the drive itself wasn't the cause of it leaving the array. If there are errors, read them carefully -- if they're command timeouts or ATA transfer or bus errors, it means a cable being loose is more likely the cause than anything wrong with the drive itself. If there is something like a bad sector in the log, then you should do a read-write test on the whole drive, and maybe not trust it anymore.
But otherwise, I would keep using a drive like that unless it starts happening again regularly, and of course keep backups of anything important (which you should definitely be doing, since RAID0 is fragile).