The first thing to do is try and identify the problem. It may not be straightforward to figure out exactly where the blockage is, so lift the trap drain cover and check:
If you can work out what the blockage is, you’ll be better able to tell if it’s something you can deal with or if you need help from a professional. Something that can easily be broken apart, like a build up of silt and leaves, would be easier to DIY than a completely solid blockage.
Being able to see and reach the blockage will make it easier to deal with, but you can still clear a blockage you can’t see/reach – it’ll just take a bit more patience and a little guesswork.
If you’re at all unsure how safe or easy it would be to attempt this job yourself, it’s best to leave it to a professional.