One of the fundamental reasons why the Roman Empire fell is the shortage of precious metals. The lack of coin is not the effect of the Empire's crumbling, it is it's cause. The shortage was both of silver and gold. Under Augustus the silver denarius was pure. Nero added 5-10% alloy. Trajan raised the alloy content to 15%. Marcus Aurelius to 25%. Under Severus, around 200 A.D., half of the denarius was base metal. Sixty years later, under Gallienus, the antonianus (the new coin) had only 5% silver. Gold coins suffered a similar fate.
There are two fundamental reasons for this shortage. First is the exhaustion of the Mediterranean mines. Second is the unilateral trade with China and India, which along the centuries became sinkholes for Roman coin.
"We already find Tiberius complaining in his day that the Romans were
giving away their money to foreign peoples for jewels, and under
Vespasian the imports from the east amounted to no less than 100
million sesterces (22 million marks) annually. In the two centuries
from Augustus to Septimius Severus, something like 4 billion marks of
valuable metals could have drifted from the Roman Empire to India and
["The Barbarian Invasions" - Hans Delbruck]
We could also derive a third one from the second, that is - the coin given to the barbarian mercenaries (as pay and later as tribute) also did not return to the Roman economy.
The continuous debasement of the currency went hand in hand with the State's incapacity to pay it's troops. The result was allowing the troops stationed on the borders to practice farming. Some emperors even allowed them to live with their wives. From the psychological military point of view, this was a catastrophe. A farmer-soldier is cannon fodder for the semi-savage, naturally war-like barbarian peoples. The result was that the emperors started hiring barbarian mercenaries, given that the Roman troops became ineffective. This was the beginning of the end for the Roman Empire, and it began in the 3rd century.
The reason why so much non-debased coin is found in Scandinavia/Britain is that the barbarians demanded imperiously pure silver/gold. The emperors had to pay them that, draining to the utmost the already scanty reserves which flowed in the civil economy. That's why you won't find pure coin on the territory of the Empire itself.
Now, to answer your question itself: When precious metals started flowing anew in Europe, it was because mines were being rediscovered, especially in Central Europe: Bohemia, Austria and Southern Germany. These mines were mainly of silver. This is the basic answer.