That happened to me last year--got to 1000 or so and then stayed with a friend for the summer who didn't have internet access at home (he used it at work, I guess).  When I started again in the fall, the idea of pushing through so many failed cards filled me with dread so I just deleted them, started over at 50 a day and then kept going at 25 per day when I began learning new ones again.  Relearning is much much easier than learning and I got caught up again in a couple of weeks.  I know others don't mind working down a large stack of failed cards but I felt that I would get frustrated by it.  Good luck either way--the kanji are still in your brain so don't worry, you will get them back quickly.

Thanks!
I haven't been away from kanji, just RTK.
I like the 50 a day rule, then 25.

At certain moment I had more than 2000 cards expired... It took me a lot of time, but reviewed them all and continued reviewing normally after that. It takes time, but you are going to review them all anyway so there's no need to worry that much about it. Something that helps you when you have more expired cards that the ones you can handle in a short period of time, is to review on a per stack basis instead of taking them all together. Give the highest priority to the lower stacks, and only review the upper ones when the first are empty.

Meh, this question was asked 1000 times here, but there was never really a good answer.
Doing rework is a really hard and painful task, but it would probably more efficient. Nukemarine's advice, on the other side, can be more fun. I'd probably try both before deciding.