Depends how big the gap is. If you think the gap is very small, perhaps try putting the retainer in (clean em first obviously) and if it simply fits 'tight' then it might be ok. If they simply dont fit or its painful, sadly your only option might have to be what a lot of us are doing - getting braces once again, waiting till your teeth are straight and then retain them properly i.e. wear your retainers properly for the rest of your life or get 'fixed' retainers on the back of your teeth.
As ever, seeing your dentist/ortho is the best and safest option.
Braces as 13 year old on NHS - put the retainers in a tissue at lunch time....lost em :(
Braces as an 18 year old (still with NHS) - idiot ortho didnt finish the treatment and fobbed me off.
Invisalign as 27 year old via private treatment + teeth whitening + fixed retainers = perfect smile...for a while. Lingual, fixed retainers eventually turned into a nightmare. Reinvisaligning myself before going to Vivera retainers. Relieved to get rid of those horrendous fixed retainers....