Sometimes the tapes show up on eBay, but they're not cheap.
Before doing any drastic measures, make sure the pad that presses the tape up against the head is ok. If you replace it and it still doesn't work, I wonder if reformatting your tape (with the NEWM command) would work. It seems worth a try since you can't read the tape anyway. There's a post about refurbishing the cassettes (actually putting new tape in them), here
, and one about the mirrors here
After someone said the tapes deteriorate over the decades, I had an occasion last year to find out mine all worked fine. I have an HP-71 with 177K of RAM, enough that I could just keep everything I wanted in the machine all the time. (That's basically true of my 41cx with double extended memory module also.) One of the 71's memory modules went down, apparently not from any hardware problem but some kind of configuration problem, and I had to re-load a lot of files. I had not even turned my tape drive on in many years, and had to use clip leads to connect a separate 5V power supply in place of the no-good rechargeable battery pack. To my delight, it worked perfectly, tape after tape.
I like to keep a printed copy of everything on the 41 (and not on thermal print paper that fades!). It's not practical of course on something with megabytes-long files, but on the 41, if worst comes to worst and you have to key it back in, it's short enough that it's not out of the question, and it's much faster than having to re-write things from scratch.