Ah, that is very helpful. Thank you.

I create a sample text file containing a single line

{{1,2,3},{4,5,6},{"ciao",e,f}}

I import using

data=Import["data.txt"];

Mathematica will often think it is being helpful by hiding many important details so I use

FullForm[data]

That shows me]

"{{1,2,3},{4,5,6},{\"ciao\",e,f}}"

Using Fullform on an expression when something does not work is a very powerful trick and it will help you often if you remember it.

This shows that Mathematica read the file not as a matrix of numbers, but as a long string in quotes.

I try to turn the string into a matrix of numbers

FullForm[ToExpression[data]]

which gives me

List[List[1,2,3],List[4,5,6],List["ciao"]]

and

MatrixForm[ToExpression[data]]

shows you your matrix.

I caution you, MatrixForm and FullForm and other "Form"s in Mathematica are good to look at the data,

but each of those has placed the information inside a "container" and new users are often confused

when they try to then do math operation on the output from FullForm or MatrixForm. If you want to do

operations on your matrix I would suggest you use

newdata=ToExpression[data]

which will give you your data in a two dimensional list, but not contained inside FullForm formatting.