Did she see some movie with a monster in it? Ask her to describe her monster. Ask a lot of in depth question about her fear. What color is the monster? Why is he or she in her room? Almost like allowing her to visualize however giving you information you can educate her about. Has she seen the movie Monsters Inc? That was a huge thing for my daughter and I think helped bypass the monster fear as that movie shows monsters to be so sweet!
Educating and empowering has always been my tool on fears. I don't ignore them and try to sympathize, however I when my daughter got this really bad fear about weather, like wind, tornados and all that I just got her some books on the subject. Explaining why tornados happen, what has to happen outside for it to happen, that weather is out of our control and warnings we get on TV and so on, she really felt so much better about it. It was a few months of doing this though.
With monsters it is a little harder. Their imaginations are on over drive at this age. Just talk to her about it. Give her a spray bottle filled with some water, so you spray all around the bed and closet. However this tricked worked for my son, I just don't really agree with it. I mean you are suppose to be showing her there is not such thing as monsters and then you are spraying for them??? Mixed messages but as I said it worked for my son.
Try to be understanding however you do have to be careful for her not to milk this as an excuse to get out of bed either.
Instead of a flashlight which requires her to turn it on and off, why not a night light? Glow in the dark stars on the ceiling and a dream catcher worked for my son too. Around three he went through this scared of monsters, gremlins you name it and we did the glow in the dark stars, my mother in law sent him a dream catcher and I did the monster spray. It solved the problem.