If you open the gauge up you have to do it in a clean area with no steel or iron trash around, and no large dust particles. Get a bright light, magnifier, and non-magnetic tools.
Meters have two basic ways they return to zero. One is with a magnetic return that pulls a movable magnet back to zero. This meters have fixed field coils and a movable magnet. The other is a mechanical return using a taut band or spring (these are more delicate and expensive, and normally not found in vehicles).
Five things can stop the movement from returning to the zero position.
1.) Trash in the movement, like a small sliver or chip of steel or dirt. Normally you will find it stuck to the magnet, and normally it is a sliver or chip of iron or steel.
2.) The zero regulator that controls the position of the return (magnet or spring) is not set correctly. There is normally an adjustment for this that rotates the magnet or spring. Almost every meter has this. The zero regulator is normally a small nut that locks down a movable piece that has the spring on it. Sometimes on a moving magnet meter it is a round flat magnetized disk, and it is moved by a nut, a screw, or simply by pushing and rotating the disk if it is riveted in place.
3.) The jewels or bearings have a bad spot or are too tight. Normally automotive indicators or moving magnet meters don't have these adjustments. The jewel or bearing tightness adjustment is normally a small screw inside a hole in the center of the nut that adjusts the return spring. You should be able to gently blow air on the pointer and it should freely move. Never over-tighten the jewels.
4.) The pointer is bent and hitting the scale or cover, or the scale is out of position and hitting the pointer or mechanism that moves the pointer.
5.) Something else electrical is telling the meter to move up off of zero. (almost forgot this one) Disconnect the battery and see if it goes to zero.
If you are careful and work gently you should be able to fix the tach. Otherwise a person who has worked on meters could fix it. A non-zero problem is usually an easy repair if you work slow, clean, and careful.