Fuel lines on the older saws 0XX (026 044 etc) are not ethanol friendly. If they have not been replaced they need to be. If it is a MSXX0 model saw such as your MS250 then according to Stihl the lines are ethanol safe but the carb is not. Never let ethanol gas for your saw sit for longer than a week, PERIOD. If you don't use it up in that time frame you mixed too much.
I use ethanol free fuel in all my 2 cycle engines. It has a good shelf life but mine rarely last more than a week often much less and I mix 2 gals at a time.
Some area it is hard to find other areas just more expressive.
https://www.pure-gas.org/Fuel lines are a place to start. Impluse line, fuel filter (pickup body as Stihl calls it)
What condition is the plug in. Color etc.
You said it ran fine a month or so ago. Sometimes a month or so ago is 1, 2, or 6 months sometimes 12 months as we lose track of time
Was fuel left in it a month or so ago?
When you stated your issue I immediately thought the following as I have had all of the below and not always the same reason at one time or the other:
1, Does it start and then die without really running?
2, Does it start and idle for few seconds and then die?
3. Does it start idle fine but when run WOT then die as it idles down?
4, When it dies is it a clean die or does it struggle as it dies?
5. Does it start idle but die when you throttle up?
Edit add on: which 2 cycle oil do you use in your mix?