sudden vision loss in left eye - advice plz

My age is 39 years. I work using computer system.
since two days i am suffering from vision loss in left eye.
two days back i felt some strain in both eyes and experienced some blurred vision some times. I dont have any vision problems till now. I dont wear any glasses.
Using this left eye i am able to see BUT the THINGS what i am seeing
are COVERED by a BLOCK or GRAY CIRCLE and the objects are not visible properly.
i also feel some strain in left eye, and water comes out often. Right eye is ok.
is it a desease? or temporary effect? or serious?? what could be the reasons?
I am worrying much about this. And one more doubt? Does dust causes this problem
when we go for cleaning or removing heavy dirty dust etc..

with thanks

MSR