Hi there, and welcome to the wonderful world of beekeeping!
it sounds like the two flat-roof hives may have been left vacant for some time after the colonies died out.
The first step would be to carefully dismantle the hives and assess each component separately. Remove any old comb, dead bees, debris, and any damaged or mouldy material. If there is old comb present, it’s best to inspect it closely, if it has signs of disease (such as unusual brood patterns, sunken or perforated cells, foul odours, or suspicious residues), we’d recommend not reusing that comb and seeking advice from your local beekeeping association or a bee inspector before reusing the equipment.
For the wooden hive parts, a good clean and dry-out is usually enough. Scrape away wax and propolis, brush out debris, and allow the boxes and parts to dry thoroughly in the sun if possible. Avoid using harsh chemicals, as residues can be harmful to future colonies. The Flow Frames and other plastic components should be inspected carefully; if they are clean and free from disease concerns, they can generally be reused after a thorough inspection and cleaning. There is some information on our website here: Do I need to clean the Flow Frames?
It would also be worth finding out, if possible, why the colonies died. Were they treated for pests such as Varroa mites (where relevant in your area), did they run out of food, or were there signs of disease?
Before installing new bees, we’d suggest having a local experienced beekeeper take a look at the equipment with you. They can often spot things that are difficult for a new beekeeper to identify, and it’s a great opportunity to learn.
If you could reply here with photos of your hives, we might be able to get a better idea of what is going on.