Worst case: Your pen drive is dead. A pen drive that has gone through thousands of Read/ Write cycles can die a natural death when its flash controller decides that it had had its full run and it is no longer safe to write any data on it.(simple non-technical explanation.
Since you have not given any details about the pen drive in question like the model/make/capacity, for how long you have been using it, whether it is still under warranty, whether other pen drives works normally on your computer,whether you had tried this pen drive on another computer and with what results, and if it shows in the Windows Disk Management how does it appear (is it RAW) etc., etc., it is not possible to give any specific suggestion to deal with that problem.
So here you go on a round-trip. How To Fix Write Protection Errors On a USB Stick