error code U0100 .no engine managment lingh or nothing on the dash and car running great

[2009joe]

hello toyota world .i thought for the craic id run my autel 802 all system scanner on my car just because i did not have any problem lights on the dash at all and the car is running great. well i was kinda surprised to see a fault code coming up on my scanner a U0100 history   loss communication with ecm/pcm  "a"    above the a there is only one coma not sure how to get that up but never the less the car is running great . what would cause this problem to come up out of the blue 

[Red_Corolla]

Most obvious one I can think of is if the Battery had been off and the last ECU to go to sleep flagged that the other one had stopped talking. No big deal if it's only a history code.

[2009joe]

7 minutes ago, Red_Corolla said:

Most obvious one I can think of is if the battery had been off and the last ECU to go to sleep flagged that the other one had stopped talking. No big deal if it's only a history code.

tbh not sure was working on the radio Speakers and class d amp upgrade might have took off the Battery and not sure when i did a scan on the car last to see if any faults come up .just thought id try that was the only one came up . but no lights on the dash and car drive great also