Water flow rate
With modern, wall-hung, light-weight gas boilers, water flow rate is particularly important.
Check that the mains water is on. If it is, check your feed and expansion tank (if you have them, they’re usually found in the loft).

Float valve
First, shut down your central heating boiler.
Locate the tanks and double check that the float valve isn’t stuck. If it is, you’ll have to move its metal arm to release it. Check that the open safety-vent pipe is correctly installed. If not, air can get into the system, causing noise in the boiler. If this is the case, you’ll need to enlist the help of a professional heating engineer.

Frozen water supply pipe
If this doesn’t work and it’s currently very cold outside, then it’s possible your water supply pipe may have frozen.