EXCEL 2007 formulas stop evaluating

I am working with EXCEL 2007 and at some point all the formulas one column of my worksheet stop evaluating. for example, I type = H3&":"&I3&":"&J3&":"&K3&":"&L3 and hit enter, and instead of concatenating the data in the referenced cells it just puts the formula as I tp[ed it in the cell. How do I restart the calculating (and how did I stop it in the first place? I hit the Esc key while I was copying a formula -- could that be it?