Pixel 5a won't turn on

Unfortunately, Mary in CA is on to what is about the only real hope here.

Assuming you've eliminated the external factors such as the charger itself, the power outlet and the charging cable, that leaves the battery, the charging port and the charging circuit on the logic board. Most of us would have to give up on the phone if the charging circuits have failed, as those kind of repairs require specific expertise in electronics and microsoldering.

Unfortunately, Google has chosen to mount the charging port directly on the motherboard rather than using a separate daughter card, so replacement of the port is particularly difficult, especially given that it's a USB-C port. Generally that requires a hot air soldering station to replace, something most of us home DIYers don't have handy.

So as far as it being a power issue, that pretty much leaves just the battery. Since your phone doesn't support wireless charging there isn't really much you can do in the way of testing; your best bet is to just replace the battery and hope that does the trick. I hate to have to suggest it, but if a new battery doesn't fix it, you may have to resign yourself to having a professional take a look at it to figure out exactly what's wrong and what it'll take to fix it.