Good afternoon, it can depend on the type of drain.
If its a grilled drain, you will probably find there is debris in it blocking it such as leaves etc.
Your best bet would be remove the cover, and see if you can fish out what in the first 30cm out (you can use your hand or a rod of some sort), as you dont really want to be pushing debris further into the drain.
The next thing I would look to do is use a bucket of hot soapy water, you want to get as much water in the drain as possible (to really flush it), if it still backs up then you need to look at using a drain cleaner (any cheap brand you pick up in the super market will work). Leave that for a few hours then try and rod the blockage threw if its still backing up.