Well yes, they didn't typically go back and fill in the mine working which could extend for miles and miles and miles. Typically they'd put a whacking great concrete plug at the top of any shafts and call it good.
But reality is that within a very short period of time many of those workings would have been flooded out quite extensively causing all sorts of damage to props and supports as well as any other equipment underground. Draining them out would be a mammoth undertaking by itself quite apart from them going in and rehabilitating the shafts and workings. Even mines in areas where perhaps water ingress was not a major concern there would still be significant work required below ground to secure the tunnels and workings and make them safe for mining activities.
This all assumes, of course, that the existing shafts can even be accessed. Many of these mines have since been redeveloped on the surface so you'd also have to re-purchase the land to access the shafts or sink new shafts to intercept the original workings.
Bringing old mines back into production could certainly be done, anything can be done with enough will and money, but it ain't going to happen. Coal, deep level coal mining anyway, in this country is done. There are perhaps a handful of edge cases where it could be possible to restart deep level mining again, the proposed mine in Cumbria where they intend to go after coking coal and are proposing to reuse some old workings for instance, but it's going to be a handful of edge cases.
Even there the number of employees probably won't be all that great. Modern mining is pretty damn efficient. You don't need thousands of men to extract large amounts of coal (or other materials) anymore. Few hundred is all you'll need to extract thousands and thousands of tonnes of coal a year.
Any political party which tries to suggest that "Vote for us to re-open the pit/steel plant/factory/chemical works/[insert other industry here]" is selling snake oil. Many of those industries closed for good reasons when they did. The failure was the writing off of those areas where those industries did close, an issue which has persisted for decades. Trusting that the "market" would sort out the towns and/or regions that had their guts ripped out when the traditional local industries went away was always ludicrous.
I get the frustration and sense of pain, loss and grief (I have lived on Teesside my whole life, we're a bit of a poster child for the issue and close to plenty of other areas that could be poster children). But pretending that if only the politicians did x (whatever x might be) we'd be back at full employment with everyone having secure well paid jobs working in real industries making stuff is actively harmful to trying to help these areas recover.
Of course part of the problem is persuading HM Treasury to invest in these areas but that's another topic (my signature refers to my feelings on the Treasury

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