I was wondering why a CD player that previously used to play CD-Rs now
isn't able to read them?

It will still play normal pressed CDs.

The normal CDs have a silver coloured surface whereas the CD-Rs have a
green coloured surface.

If it's now failing to play specific CD-Rs that it used to play, then
this could indicate that the player's laser diode is getting near the
end of its life. Its output is dropping, resulting in a weaker signal
reflected from the disc's data layer, and the photodiode in the reader
assembly isn't getting a strong enough reflection to track the data
spiral and/or decode the data bitstream.

The same effect could result if you've got a buildup of dust or
tobacco-smoke tar on the laser diode or the photodiode. The more
light is blocked or scattered by these materials, the weaker the
detected signal.

CD-R discs already produce a weaker reflected data signal than an
"pressed" CD. CD-RW discs are (usually) even weaker. So, a CD player
which is "dying" will usually being to reject CD-RW discs first, then
CD-R, and then finally "pressed" CDs.

If it's a CD player of particularly high value, it may be possible to
have the optical assembly, or even the entire "loader" mechanism
replaced. For cheap CD players, alas, the cost of servicing them is
usually more than the cost of buying a new one (if the service parts
are even available).

If the problem affects newly-burned CD-R discs, more than older CD-R
discs you made years ago, then the newer CD-R blanks may be to blame.
Some brands and types of CD-R disc have poor reflectivity, and the
"80-minute" CD-R discs (which are pretty much the only kind you can
find these days) are inherently harder to track than the "72-minute"
variety which used to be the standard.