Purchase or borrow a lens cleaner disc and try it first. You can find
these things really cheap at many stores. They usually
get the job done well. However, you may need to clean the lens manually
if the store-bought cleaner fails.
OR
Use a cotton swab and some isopropyl alcohol to gently wipe the lens.
Just a quick scrub will do. Dry the laser pickup lens with the other
end of the swab.
Give the lens a few seconds to dry and replace the covering of the DVD
player. Test the efficiency of the player. If necessary, repeat the
process.
Of course it may not be dirty lens problem at all but something more serious, but it's worth a try.