Can't login to Admin panel on Mac

This week I suddenly lost the ability to login on the Admin panel of WordPress via any browser on Mac. Although I input the correct username and password, I get a loop: the same login screen loads over and over.

Recently my web hosting account transferred from Yahoo! Small Business to Aabaco; however, the site is up and running, and i am able to log in to Admin panel on a PC/Windows machine.

any insights?