Hello, and welcome.
More elboration would be nice... but from what you have given.
Without being too insulting I take it the cells are fresh ones (check or confirm).
When you turn on and you flick the switch try all the way. The power switch may be flaky (perhaps from lack of use there is oxide present on the contacts within). Try flicking the switch in a positive manner a few times, and give a moment for the result to register. I have a particular NLU, that can stick like this sometimes, it happens once in a blu moon (stupid pun intended).
Surely hope this works, and you do not have further issues, which will require more detail in what you have before you.
Good luck.