question

MD and PA have tax reciprocity.  That means your W-2 income earned in MD is taxable only by your home state of PA.  Your employer was incorrect in withholding MD taxes.

First step: file MD Form MW507 (completing line 5) with your employer ASAP, to get them to cease withholding for MD.  Here's a link to the form:

https://www.marylandtaxes.gov/forms/current_forms/mw507.pdf

In order to correct the situation, you must 1) file a non-resident MD tax return on which you show the MD withholding but allocate zero income to MD; 2) file a resident PA tax return to which you allocate ALL your income.

This should result in your getting a full refund from MD, but having a tax bill to PA.