What is a loose neutral? What do you do to fix it? Thanks

You home has 4 wire, ground, 2 hots, 1 neutral. To get 120 volts you get power from one of the hots and a Neutral, if 1 neutral is for 2 circuits, and it become loose, or burned open,
Things on these 2 circuits could go as high as 240 Volts, and destroy 120 volt stuff.
If it is your Main Neutral, it will affect the whole house.
Neutral is marked with white tape.
Power needs to be turned off, neutral tightened at Panel, Disconnect, Meter, wherever it is loose. Verify all.