What sort of text?

Where is it entered?

What type of numbers?

A small example would be nice.

Gord Dibben  MS Excel MVP

It sounds like you have the data formatted for numeric in the first row of
that column.

Select the problem cells, right click, select format cells, and select
Number type of "General".

Good Luck!

Do you mean "what do I do now?", or do you mean "how do I avoid this in the
future?".

For "what do I do now?", the answer depends on whether or not the "numbers"
were changed when you entered the text.  If the displayed cell contents look
exactly as you intended (except perhaps for alignment), you can simply change
the cell format to Text by clicking on Format > Cells > Number and selecting
Text.  However, if the "numbers" were changed (e.g. a 20-character string of
digits is changed to a rounded value usually of the form 1.23457E+17), it is
too late to correct the situation.  You must re-enter the "numbers" as text;
see the next paragraph for instructions.

For "how do I avoid this in the future?", there are two ways.  IMHO, the
"best" method is to type a single-quote (') before the "number"; for example,
'12345678901234567890.  The single-quote will not be displayed, but it is
part of the cell content.  It tells Excel to treat the subsequent characters
as text, no matter what they look like.  Alternatively, with forethought, you
can format the cell as Text before entering the "number".  In my experience,
that is error-prone because eventually the format will be changed unbeknownst
to you because of some action that you take (e.g. deleted the column or row
that contains the cell and forgetting to reassert the Text format).  On the
other hand, it might be the only way to ensure that cut-and-paste data
(devoid of the single-quote prefix) are treated as Text.

HTH