Tuning your instrument when it gets really out of tune can be a challenge, especially if you need to use the pegs!
First, you need to have some sort of resource for checking your tuning. This can be:
1) An electronic tuner. Here are some good ones you can purchase:
2) Tuner Ninja
Here is a GREAT video that walks you through the process of tuning your instrument step-by-step: How To Tune With Your Pegs
Worst-case scenario: If your local string shop is open, you can always stop by and have them tune it for you :)