Sewing machine pokes fabric but string won't stitch in fabric?

If needle was changed make sure it was replaced correctly 

As has been suggested, re-thread your machine. Also, try putting in a new needle.

if the bobbin isn't threaded in correctly it won't work. Check your manual for the sewing machine and see if that's the problem. Or just watch YouTube videos on how to fix a sewing machine. Best of luck to fixing it!

Re-thread your machine.  I went through this, recently, in a sewing class.  The 'new' $17,000 machine would NOT stay threaded.

Re thread both upper and lower. Always have the needle uppermost when threading, as it messes up the tension if it's down. Make sure the bobbin is feeding in the direction it should be for your machine, usually clockwise. A new needle may be needed. Clean the bobbin area with a soft brush, lint can cause all sorts of problems.

Great answer on one i read.

Yes, same threads

Rethread

Check or adjust tension 

Make sure the bobbin thread and the needle thread are the same type of thread.

Unthread the machine and remove the bobbin.

Make sure the needle is the right size for the fabric you are using.

Rethread the machine, double-checking that is threaded correctly and that the bobbin is in its case unwinding in the correct counterclockwise position.

9 times out of ten problems with sewing is because of not threading the machine correctly.

so when this happens rethread your machine. Check your bobbin case for stray threads. Make sure the bobbin thread pulls out smoothly. Rethread the upper thread.  Make sure the pressure foot is UP. Once threaded take both upper and bobbin threads and pull them behind the needle to start.

This usually happens when the top thread is threaded through the machine wrong or the bobbin is installed wrong.  First check that the bobbin is wound and seated correctly.  Secondly, re-thread the top thread through the machine, making sure that while you do this the presser foot is in the UP position. If the presser foot is not in the UP position, the thread cannot pass through the tension discs inside the head of the machine, and you won't be able to sew correctly.