PS ....
If you are trying to enter a number with more than 15 digits, you are
exceeding Excel's data entry limit. It recognizes only the first 15 digits.
I should have said 15 "significant" digits; leading zeros do not
count.
Some other common problems:
1. Be sure that the cell is formatted either General or Number. Click
on Format > Cells > Number.
2. If you are entering "long" numbers and they are changed to numbers
of the form 1.234E+14, change General format to Number with an
appropriate number of decimal places.
3. If the length of your numbers varies widely, you might try
Scientific format with a large number of decimal places. But that
probably does change the appearance of your numbers, unless you are
entering them with scientific notation.
Note that all of these formatting ideas only change the appearance of
the number that you enter, not the true value of the cell.