Dear Albert,

try

ClearAll["Global`*"]

That clears the global context. The documentation shows how to clear private contexts.

instead.

I suppose that this is not really a matter of "who the heck knows why just be happy it's working again". If you open two notebooks they will use the same kernel unless you explicitly specify using another kernel. That means that variable assignments in one notebook affect the definitions in the second notebook. As Gianluca says, RecurrenceTable does not protect its variable names.

In your case the n was probably somewhere defined to be 1. If you then want to use it as an iterator, it will (before the evaluation) be substituted by the value 1. Then the recurrence table does not have a valid iterator variable and does not work. Once you kill the kernel or clear the variables it will work again.

Cheers,

Marco

PS: If you post the two notebooks we can probably trace the problem down more directly.

Killing the Kernel is overkill. It is enough to Clear the variables you are using inside RecurrenceTable, which does not protect its variables:

n = 1;
RecurrenceTable[{a[n + 1] == 3 a[n], a[1] == 7}, a, {n, 1, 10}]
Clear[a, n];
RecurrenceTable[{a[n + 1] == 3 a[n], a[1] == 7}, a, {n, 1, 10}]

Works fine on my machine:

Have you tried restarting the kernel?

Best wishes,

Marco

PS: Sorry. Your second message came only up after I posted my reply.