Why is my HP pavilion 21 not powering

Check the power supply with a multimeter first - these use laptop supplies and they're prone to burning out from 24/7 use on a machine like this. These use 19.5V supplies like a laptop does so it shouldn't dip past 19V under load. Now that said it's fine for it to dip to 19V without load and run at 18.5V under it will work, but if you load it heavily it will have problems. As such I generally consider 19V (no load) on a adapter used on an AIO to be "replace" voltage to avoid issues. This is probably the cause of failure. Get a 90W adapter even if it can run on 65W as it will give you mote headroom for issues like this.

If that doesn't help I would take the machine apart and test/replace the CMOS battery. A lot of HP products from this period are sensitive to the battery voltage and do things like this. If it is at 2.5V you have to replace it with these. Use this guide up to Step 3 to access the battery on the motherboard: HP Pavilion 21 TouchSmart All-In-One PC Rear Chassis Fan Replacement