You can drop back to a previous version of PHP if that's what has caused your issue, but I wouldn't recommend it, not least of all due to security and code changes.
Well, yes - technically.
As it's your site, you maintain responsibility for ensuring any changes you make are going to work with the site or application you are having hosted.
Granted, had I had a site built for me, I too would expect things to just work, but as I'm in the business of development and support,
I know all too well that things rarely go to plan, try as I might to avoid such issues.
If the work carried out by your dev had been done in a way that allowed for future upgrades, then yes,
the PHP upgrade would likely have made things better.
However, as Andrew has already said, the original dev left things in a rather unsustainable way, meaning that future upgrades to PHP/MySQL
would have required additional work in order to maintain a stable site.
So either recruiting a new dev to take over maintenance of your site, or locating your wayward dev to redo some incompatible code would have
been the better course of action for an individual with little or no experience of their own.
It just seems that on this occasion, a series of factors that no one in the current circle of your site could have foreseen, resulted in your site going down.
It's not really anyones fault, I just would have checked compatibility first, or left it as is