Why the headphone jack on my Lenovo ideapad would stop working all of a sudden?

1. Have you tried other headphones and do they work?

2. Are all your drivers up to date and installed?

3. Do your headphones work on other devices?

4. Are the headphones being detected by Windows?

If 1 is false, try other headphones. If they work, check 3. If they don't check 2 & 4.

If 3 is false, it may be your headphones. Check 1 if you haven't already.

If 2 is false, update your drivers and install missing ones. Then go to 1. If 2 is true, check 4.

If 4 is true, check 1. If 4 is true, check your sound mixer.

Conclusions:
If 1&2 are true, and 3&4 are false, it is probably your headphones.

If 2&3 are true, and 1&4 are false, it is probably the headphone jack. If it is still under warranty, you can send it in and they will fix it for you.
If not, you have 3 options:
- Bring it to a computer repair shop. They may be able to fix it.
- Buy a USB soundcard. (Recommended)
- Buy a USB Headset

If a cold reboot or a scan disk won't fix the problem, then its prolly a sound card or sound program that is causing a conflict.
It could be something you recently loaded.
At this point you would then have to troubleshoot the sound card and its software.
Which is a lengthily process. ( removing and reinstalling etc )