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.

Use your last aligners for a few weeks.  Chances are they will move your teeth back and get you on track.  Worth a try and cannot hurt.

Unfortunately, teeth do not stay in place if you stop wearing your aligners and the next aligner assumes your teeth are exactly where the previous aligner left them.  Going a month without wear is almost a guarantee of failure for the new aligners to fit.  All you can do is get some retainers to hold your teeth in their current position and then new Invisalign molds and reprogram your treatment!  We tell all of our patients this and STRESS what will happen  if they do not follow the program!