I’m a newbie in this and out of curiosity, I opened the back of my Olympus mju zoom 105. The number then went back to 1, are my prev shots ruined? And if it is, do I continue taking from number 1 or do I have to do something to make the number go back to where it was?
You know the film canister? And you had to pull the film leader out a little to load the film into your camera? Well, any film that gets exposed to light is ruined. Film is light sensitive. Extremely light sensitive. The photos you have taken so far have been lost. The film that remains in the canister is still usable. (The canister is designed not to let light inside.)
I am kind of guessing here, but I think when you closed the back of your camera, the camera assumed you just loaded a new film. You can go ahead and take photos now. The camera will have advanced the film a little so that the frame now positioned behind the lens, waiting to be exposed, is film that was just drawn from the canister since you closed the back. The camera has reset its counter, so it will be counting from 1, but you will not get 36 frames from this roll (or 24, whatever was written on the packaging), because there's that strip of ruined film already wound onto the take-up spool (that is, those first frames you shot, which were ruined when you opened the camera back).
Be very careful not to expose film to light. I'm still a little in awe of how people can do this, but I guess if you are never told...
Load the film carefully, don't open the camera back, unload carefully, and bring the film to the photo processing lab without pulling it back out of the canister.
You've certainly fogged the frame that was in the film gate, and the ones before and after it. Most likely, there will be some damage to film that was already wound up on the takeup spool as well, though images near the start of the roll may have little enough damage to be salvageable. You can, if you wish, finish the roll, but you may run into complications.
There probably isn't any way to restore your frame count on a counter that automatically resets, and that's okay -- but depending how the mju Zoom 105 manages end-of-roll rewind, you may find that it won't rewind when the film comes tight, or will only rewind to the point in the roll where you opened the back (the new frame 0). Assuming it does rewind automatically, it may also be difficult to tell for certain whether it rewound to the "start" (new frame 0) or all the way until the film comes off the sprockets.
If you don't hear the film disengage from the takeup when you rewind, you might need to open the camera in a darkroom or changing bag, verify if the film has been wound all the way back, and if necessary hand wind the remainder of the film into the cassette (this is pretty easy).