What usually breaks is the door open lever.
On
a counter top model, the outer cover will need to be removed to
diagnose and repair this. On an over-the-range model, only the control
panel needs to be removed.
Please unplug the oven first.
If
you or someone you know wants to look into this, we have critical
safety, disassembly, and troubleshooting info at our site, and our link
is at
our listing here on FixYa.
Once
it is apart, you'll be able to see how the door open button would press
against the door open lever, which would then lift up against the door
latch heads (hook-like projections on the door) and the door should
open.
To order parts, refer to the Sunbeam number in your manual.
We're happy to help you with free advice and we'd appreciate your thoughtful rating of our answer.