If you have not done so, restarting the iPad is always worth a try.
Hold down the power button until the red power off slider appears, and power off. After the iPad finishes shutting down, hold down the power button again until the Apple logo appears.
Besides outright glitches, the grey screen appears for several reasons.
Sometimes is it simply incompatible content from the website. This isn't all that common, but if you're browsing habits happen to take you those sites a lot, then I might be often for you.
Some types of popups do this. To prevent this go into Settings app >> Safari, and disable popups.
I've had advertisements do this. The kind that cover all or most of the web page, but for some reason don't count as popups so they are not blocked. When this happens a careful examination of the screen (you may have to scroll to the top right) shows an standard x-in-a-box to close. Tapping on it will usually close the advertisement.
And other times it just happens. All you can do is restart and try again. For most of us it's a rare thing. I'm hoping a restart will turn it back into a rare problem for you.