Blocked ears during/just after a viral illness (cold, flu etc) is common and nothing to worry about long term. If you are unable to clear your ears then you should not fly in any situation where there may be pressure changes requiring you to clear your ears. Low level may be OK but any pressurised aircraft should be avoided. The main problem is that you only get ear pain on descent when it's too late to do anything about it!
Syringing ears will not help this sort of thing as the blockage is in the middle ear and syringing only treats the outer ear (as the eardrum seals one off from the other).
If symptoms do not settle after about a week/10 days after the viral illness has settled then you should probably see your GP. Likewise if the symptoms get worse.
Hope that helps
Bob