I find it very difficult to decide whether I should use plural/singular noun after the word "no". For example, some friends told me we should say "No problem", but some others said it's correct to say "No problems".

I only saw "No Dogs Allowed" but not "No Dog Allowed" before.

There are a lot of confusing cases. Look at the one below:

(1) There is no photocopier in my office.
(2) There are no photocopiers in my office.

Which one is correct? Singular:(1) or Plural:(2)? Or both? Could someone help?