Just so I understand correctly, you are setup with the "Push" for Gmail settings through an Exchange setup?
Is your calendar the only thing that you get pushed to you, or do you get mail and contacts too? If so, are they working?
Generally, if you are using google's exchange setup for push services, and it stops working, its on Google's side.
Here is a way to test:
On your phone, go into Settings and then Mail, Contacts, and Calendars and turn off the account.
On your PC, log into your calendar and make a new event for tomorrow. Verify it's in there and log out.
Back on the phone, turn the service back on. Check to see if the new event shows. It should come up within a minute (you can look for the little rotating icon up next to your carrier symbol to see if it's even trying to pull down the data)
If it fails, then you have to look at what could be different. Changing a password on the PC will definitely affect it, since they share the same account login info. Also, if the server info has changed, that will also cause problems. You SHOULD get an error letting you know whats wrong, but sometimes that doesn't happen.
BTW, when I say PC, I mean online.