It's possible you have some water droplets in your lawn mowers gas tank. Even a few water droplets can cause your engine to stall.
It's also possible you could have a partially clogged pickup tube or the main jet is clogged in your lawn mowers carburetor.
If you didn't run your lawn mower out of gas before storing it for winter and the gas turned to varnish it could cause these problems even if you placed fresh gasoline in it.
I recommend running your lawn mower out of gasoline and then putting some stabil in your gas tank over the winter.
If your carburetor main jet is clogged you'll need to remove the carburetor and soak it in some B 12 chemtool parts cleaner and then wash and blow it out.
If it's the kind of carburetor that sits on the tank then you'll just need to remove the carburetor from the tank and clean the pickup tubes. You'll need a new gasket between the carburetor and tank to put it back together.