Ah... You tried to take the bolt apart (or somehow unscrewed it unknowingly) with the safety in the fire position. Or maybe you took the safety off when the bolt was out of the rifle and disassembled. I’ve done this before to my M70 Pre-‘64 Winchester, which is very similar to your Mauser. I’ve disassembled my friend’s Yugoslavian M48 before and it was similar enough to my M70 that I think I can help you.
Don't move the safety when bolt is out of the gun unless you know what you're doing.
There are 3 positions to the safety: a fire position, a middle safe position, and a far-right (I think it's right not left) safety position. The far-right position locks the firing pin back, but does not allow the bolt to open. The middle safe position locks the firing pin back, but it allows the bolt to open. The fire position lets the firing pin rest on the bolt cam (I just made that name up) when the bolt is open or the trigger catch (another made up name) when the bolt is closed.
When taking out the bolt it is a good idea to put the safety in the middle safe position. That way you can open and remove the bolt and keep the firing pin locked back and secured by the safety mechanism. This allows you to unscrew the back of the bolt and remove the firing pin. The safety mechanism is the only thing keeping the firing-pin-spring compressed at this point so don’t mess with the safety or you will be in the position you are in now.
Now, about how to get your gun back together: The firing-pin-spring is decompressed and the bolt is out of the rifle. If the bolt were in the rifle, all you would have to do to recompress the spring would be to rotate the bolt up. When the bolt is in the action of the gun, this is very easy because the action is made of steel and has a good grip on the bolt. When the bolt is out of the gun, your soft hands must try to do the same thing. Depending on how new and strong your spring is, this may be a challenge, but on an old surplus rifle I hoping it will be a piece of cake. Now before you go twisting on it, look for a little spring-loaded catch on the firing pin assembly (the back part of the bolt) near the interface between the firing pin assembly and the front part of the bolt. You will not be able to twist the back part of the bolt all of the way without pushing this catch in and letting it snap into a recess in the front part of the bolt.
So go ahead and try and turn the back part of the bolt about 90 degrees. Turn it in the same direction you would if the back of the bolt were a screw and the front of the bolt were a nut. If you look near the trigger catch on the back part of the bolt, you should see a sloped cut. This cut is the cam that cocks the firing pin and compresses the firing pin spring as the bolt is turned. As you turn the back of the bolt, you must push against the can and compress the firing pin spring. When you turn the back of the bolt, try not to get leverage by gripping the safety lever. I don’t know how much force the safety mechanism will take.
Turn the back of the bolt until it rests in the little notch at the top of the cam. Remember to compress the little spring-loaded catch or you won’t be able to turn the back of the bolt to this point.
Insert the bolt back into the gun.
That should fix it.
I wrote this while looking at the bolt of my M70 Winchester and trying to remember the differences on the M48, so if anyone sees any errors, please correct me.