Why is the first string of the guitar not tight, it's very loose, and it won't spin after a few turns. Please help me to figure it out, the string knob was changed and it doesn't work.

This problem may be due to a problem with the worm gear that controls the first string. Here are some possible solutions:

1. Replace the complete worm gear that controls the first string. This may require opening the bottom cover of the string knob and removing the worm gear for replacement.

2. If you don't want to replace the entire worm gear, you can try to remove the stuck part of the worm gear with a pallet knife or other tool to allow the string knob to rotate smoothly.

3. If neither of these methods work, you can try adjusting the tension on the first string. Try tightening the string knob, but not so tight that the string crumbles. If the problem persists after adjustment, a more in-depth inspection or replacement of the string knob may be necessary.

Please note that it is important to turn off the power to the guitar before performing any repair or replacement operations to prevent accidental electric shock. If none of the above methods solves the problem, it is recommended to seek the help of a professional guitar repairer.