gearbox fault!

would seem the tale of woe is set to continue,sat in traffic today snicked into neutral and heard a rattle give it a few revs cos it sounded like exhaust rattle heard nothing,put it back in drive and gearbox fault came up it drove in 4th i think,switched off and back on and all was well,yet to check fluid level cos its hot,what could this be anyone had anything like it?

could be a low battery? usally throws up this fault when the battery is getting old. i would check the battery but like you said i'd check the trans fluid first then check the battery.