I found a small bug :s
@Before
I was playing the muted version. I had an item I could upgrade (Enchanted Pole). I clicked the upgrade button to view what it would become, but eventually clicked the back arrow.
@Bug
After clicking the back arrow I was unable to equip any item, unable to start a level and unable to hire a hero.
@After
I went to the homescreen and saved the game (successful). After that I refreshed the page. The game started as normal and I was able to perform all actions again. So refreshing the page seems to fix it for now.
 "Road Of Heroes"){kind=icon}
 "Book of Mages: The Chaotic Period"){kind=icon}
Thanks! Will fix.