My footjoint has stopped playing. I can't play lower than a d. Can anyone tell me how to fix it?

Yes I do! Take it to a good technician. Don't make things worse than they already are...

Unfortunately I don't have the time, I have a performance tomorrow and don't have another flute to play. I can do some repairs with instruction though if its a simple problem, like if a key is bent.

If D plays but C# doesn't, something is up with the C# pad or key. Pad loose? Key bent? Maybe the C spring came off the hook and it is not staying open, but the low C should play. You just need to take a close look to identify the problem. Unless its just a spring thing, getting it fixed correctly could be a bit tricky if its your first repair.

I think your choices come down to having a flute that doesn't play those notes, versus a flute that doesn't play at all. Trying to fix it yourself could really be a bad idea. Given two unfortunate choices, I'd try to work around the problem and figure out how to get through your pieces without playing those notes.