“I lost a substantial amount of money.”
You owned a lot of stocks, but when the economy started to suffer, you lost a lot of your money. Now you're talking with a friend about the economy at that time. You say:
I lost a substantial amount of money.
You "lose money" when you invest in something, like stocks or businesses, that then become less valuable:
At one point, he had more than 100 million dollars, but he lost it all through bad investments and poor management.
You can also "lose money" by gambling.
The word "substantial" means something between "some" and "a lot". For example, if someone says they lost "a substantial amount of money", that probably means 10% or more of their wealth. Another example:
I earned a substantial amount of money as a teenager through buying and selling comic books at different trade shows and conventions.
Here are some other things that you can have "a substantial amount of":