Hi, I have had my D3100 for about a year with no issues.
However, a couple of days ago I was experimenting with
settings and since then, I am unable to delete photos from my
camera. Could I have done something to lock the camera up to
prevent me from deleting photos? Pls. help..
When viewing the images stored in the memory card on my D5100, pressing the AE-L/AF-L button will lock the image currently on display. You can tell this by the key icon that is displayed on the screen when viewing that particular image. When this key icon is displayed, the image associated with it can not be erased, even when re-formatting the memory card using the camera.
Try putting your camera in image playback mode and see if the images have a key icon along the top. This would mean that they are locked. They can be unlocked by pressing the AE-L/AF-L button as each image is displayed.
As Alan (kukui1967) mentioned, you may have "delete protected" your images. I'm not familiar with your camera, but on a D300 you have to press the "protect" button to protect an image from deletion. You would have had to protect all of them individually to protect the whole card from deletion, i.e. I know of no way to select them all for protection at once. Also, with a D300, formatting the card will delete the protected images.
Do you have another card you can try in the camera?