I don't think it's accurate to say there was no industry. There was nothing like the kind of industry we now know but there was industry. Especially toward the later Middle Ages (but it always existed). The increase in industry and trade led to the Renaissance rather than the other way around. (There were also joint enterprises for profit and partnerships protected by law in the Middle Ages even if most of them couldn't be considered corporations, although there are early examples of corporation like entities going back to this period.)

For much of the Middle Ages land was wealth as far as aristocrats were concerned. First and foremost, this means agriculture. The more land you had the more you could farm or have livestock. And the more excess produce you had the more you could sell or trade for it. Which brought in money.

Take England as an example, which is hardly the most fertile land in Europe, there are areas where grain grows relatively well. But other areas that are poor for agriculture but good for grazing animals. The places that had grain would trade or sell that grain. In return they'd receive or buy fish or livestock or animal byproducts like wool or dairy. Through out the rest of Europe there were fertile lands or lands where specialty items grew.

There were always niche areas. For instance, England produced a lot of wool because there was a lot of land that wasn't suited for much more than letting sheep graze. That wool would be traded with Flanders or other areas to be turned into cloth. Some areas in France or Italy were famous for growing grapes that led to the production of certain wines. Other areas were known for quality olive oil (always a high premium for that at least in the Mediterranean).

Some made their money off of mining, which is also tied to the land. Lead, iron, tin, and silver were always in demand through out the middle ages. In some places in the Middle Ages, especially in areas in Germany or Scandinavia, the nobility might make money off of vast forest area on their land, which would become necessary for timber especially as ship building began to grow as an industry. (In England much of the forest was owned by the crown but this wasn't true everywhere in Europe.)

The aristocracy may not make much of their money off of trade and manufacturing (although some did) but these areas existed. Towns in Flanders, France, Spain, and through out the Germany were well known for certain industry like cloth weaving, ship building, pewter working, weapons, olive oil pressing. The aristocracy may not be directly linked to this industry but depending where they were they may have the right to tax, or they may be charging rents, or they may be providing the raw goods for these tradespeople. So, it all led to accumulation of wealth.

As much as the concept of the Dark Ages makes people think everyone was isolated, there was pretty much always trade through out Europe. The Vikings, for instance, may be primarily remembered for their raiding. But they also had trade networks through out the Baltic. The Hanseatic league took over for them in the Baltic late Middle Ages. Italian cities such as Milan and Venice had strong trade links with the East. Trade definitely increased as the Middle Ages wore on though. And trade always provides the industrious with ways to make money. There were people dedicated to importing wine in places like London (Chaucer came from such a family) as well as oils, spices, furs, etc. The aristocracy may not be directly involved in trade but they are buying and selling things that were being traded (their land might be providing the grapes for wine, or olives for olive oil, or grain, or wool). So the increase in trade would line their pockets even if indirectly.

As the Middle Ages wore on and the feudal ties broke down then aristocrats might make their money off of rents as their land might no longer be tended by serfs. This could lead to direct cash in pocket. There was also a practice, at least in England, in giving certain lords monopolies. They would get a monopoly in, say, olive oil. And then all olive oil importers would have to give them a percentage of what they earned (this was obviously not too popular with the people who actually did the work).

And, of course, many aristocrats didn't accumulate much wealth. They might be sitting on a lot of land without any actual capital to work with. That is why marriages between the bourgeois and the nobility slowly started to become more common (not without controversy). It gave the nobility a cash infusion while it gave the bourgeois legitimacy or access to political power.