Ender 3 Marlin - Incorrect temperature
When you have updated to Marlin, you were supposed to configure it carefully. This is maybe laborious and sometimes difficult procedure, but very important. Basic guidelines are described in Configuring Marlin official guide.
I assume that you obviously did it, at least in some part to enable BLTouch.
You should review the Thermal Settings section and update it with valid thermistors types for bed and hotend in Configuration.h:
#define TEMP_SENSOR_0 1
...
#define TEMP_SENSOR_BED 1

The list of available predefined settings (numerical identifiers) is just above these #define macros. The optimistic assumption is that you know that hardware details. Popular value is 1 for EPCOS 100k (older repraps) or NTC 3950 100k, but there are many examples of troubleshooting and advices on Internet. For more exotic temperature sensors you should set value 1000 and specify own details in Configuration_adv.h.
If all these settings are correct, then possibly your thermistor is faulty. There is always a chance of coincidence with some damage unrelated to last upgrades. You can follow these instructions to verify it.