I just replaced one 15 amp breaker which was straight forward, and I replaced a 20 amp breaker with a 20 amp GFI BREAKER. Everything seemed to be working well - I celebrated with a cold beer and started to continue replacing outlets and switches... but there was no power to the house. Now the 150 amp breaker trips each time I try to reset it. I replaced the 20amp GFI With the original and. Checked all connections. Any ideas about what's wrong...

It's very possible you just have a weak/worn out main breaker. Hopefully others will offer some advice on how to deal with this.

What was the result after you removed the gfi and installed the original 20 amp breaker? By the way, beer and electrical work are not a good combination. Your description of the sequence of events and results basically gives no clues. You make changes to circuits controlled by the 150 amp breaker and then suspect the 150 amp breaker as the problem? Was everything working before you started messing with it? What was your goal when you started?