i suspect the main 100 amp breaker out side may be the problem but it may also be a problen in one of the upstairs outlets
Focus

Did you reset the main? It's not the upstairs outlet.
You need an electrician now.
Not yet.
1. Reset your main breaker by turning it off, then on
2.Call your power company.
THEN you might need an electrician. Always call POCO first.
To help you understand, you have
two hot legs @1 120 vots each, 240 volts between them. You have a bad connection somewhere in one of them. It could be the wires, the breaker or the internal compnents of the panel like the lugs or bus bar. The "silver" wire is the neutral/ground.