I'm assuming this is a true comma separated ASCII file that can be opened in Notepad
As a test, carve out a few lines (±10) from the file and save as a separate CSV file, make sure there are no header lines and see if this will import. Keep adding portions to this smaller file until it fails.
Note: You can have headers but you must remember to add a # before them then use the invalid character feature in the import type and set it to match (#).
It is simpler not to as Rick suggests but sometimes you need them to jog your memory.
The point import can be ropey with some characters, I have gone as far as to open the txt file and remove all Spaces using find & replace to get the import to work. If a number has a space before it it will throw an error.
Also if you have opened edited and saved this in Excel as a CSV file (and maybe other types). Edit the file in notepad and save it again. I have found that sometimes excel messes with the formatting or file settings but notepad will fix this.
Civil3D 2013 / 2014 British UK Windows 7 Professional 64-bit Intel(R) Core(TM) i7-3820 CPU @ 3.60GHz (8 CPUs), ~4.0GHz With 32768MB RAM, AMD FirePro V4900, Dedicated Memory: 984 MB, Shared Memory: 814 MB