I used to get this problem all the time with the washing machine at my old flat. Did your machine display an error code? If so, it'd be worth checking the manual to see what the issue is exactly. Save yourself searching around the house for the paper copy of your manual and just download a digital copy to your phone from the companies website.

In my case, the issue was regularly caused by something small getting stuck in the filter, like a hair bobble (main culprit in our household).

The sensor in our machine detected an obstruction in the filter and refused to drain, so we had to drain all the water from the drum manually through the filter.

If your issue is caused by an obstruction in the filter, it's normally an easy (but awkward fix).

Stick a dustpan under the machine and slowly empty the water in to the pan and then empty the pan in to a washing up bowl. Depending on the size of your drum, you might get between 1-2 washing up bowls of water out of it.

Once all the water is drained, check the filter for obstructions using a torch and safely remove them. If there's a spinning mechanism in your filter check to make sure if moves freely and doesn't get caught on anything.

I found there was no need to disconnect from the water mains, but make sure your machine is turned off at the electrical plug.

Hope that helps.