Mine stopped working also!!
Does it switch from fast charging to normal charging every 2 to 5 seconds going back and forth ?
Have you recently had a Samsung update to your phone ? The October update ?
Do other phones work when connected to the wireless charger?
If not, then it could be one , or more, of three things:
You should be able to test the cable and wall adapter easily. If they both work, then it points to the wireless pad. In this situation, you would need to replace the wireless pad.
There appears to be a fairly strong thread here which indicates that (as a result of recent firmware changes) S20 devices will no longer charge from certain wireless chargers. iOttie is a specific brand which is experiencing this issue -- when they are contacted, they are describing this as a Samsung software problem and referring support to Samsung. Other phones charge properly on iOttie chargers. Something clearly has changed which affects the sensitivity of the Qi charging process on the S20. This shouldn't be that difficult for the developers to sort out as it appears to be directly connected to the recent software updates.