magnetic fields - Solenoid vs coil of wire

This may sound pretty basic, but:

I'm studying for my AP Physics exam, and I am confused.

Apparently, the formula for the magnetic field in the center of a loop of wire with $n$ turns is:

$B=\frac{\mu_0NI}{2R}$.

However, the formula for the magnetic field in the center of a solenoid of length $L$ with $n$ turns is $B=\frac{\mu_0NI}{L}$.

Those two formulas are clearly not the same, but I do not see how a loop of wire with many turns is different from a solenoid. Both consist of current-carrying wire tightly wrapped into many coils, right? (Well, I get that the loop of wire is not necessarily tightly wrapped, but I am not aware of anything that says it cannot be.)

What's going on?

Thanks.