I haven't been wearing my Invisalign for more than a month now and I want to get back on it so what should I do

Talk to your dentist  first but typically, if it's just been a few weeks you should be able to resume a delayed schedule with Invisalign with the last pair of aligners that were given to you. Remember that the optimum time to wear your aligners is 23 hours a day, every day. 

If
you only had them off your teeth for only one month or so, you might still be
able to go back. If you wait much longer than that chances are the trays will
stop fitting all together. You should see your dentist before doing so just to
make sure you are not moving your teeth in the wrong direction

The first step is to speak to your dentist/orthodontist honestly. Why did you stop wearing them? What makes you think you won't stop again? He can help you decide if an old aligner will still work for you.