You've not stated which browser you're using. If it's Internet Explorer, follow this route:
Tools > Internet Options > Temporary Internet Files > Settings.
Under 'Check for newer versions of stored pages', you''ll find 4 options. The default is 'Automatically' (although it sounds as if your system might have changed to 'Never').
If you browse websites which can change almost minute by minute, you might find 'Every visit to the page' will be best for you (but this does slow down browsing).
If, however, you check on a page just once after you've started IE, 'Every time you start Internet Explorer' might be best.
When you've made your selection, click 'OK', then 'Apply' and 'OK' again.
Experiment and see what's best for you. If you don't like the way your browser works, you can always change the settings back again.
Chris