won't open some files in excel

[venkat1926]

windows 10 recently I find some files are automatically saved as <xlsx> files and some as <xls> file. I am able to open .xls files but not able to open .xlsx files. when I click those files it gives error and the error message at the top is "(folder name) not responding" and do not open. I am super senior citizen.  Is there way to open such  files or convert them as .xls file. I am used to operate excel but not savvy in correcting errors in computer. the solution should be easy to follow for a non expert. 

[#2]

@venkat1926 Which Excel version are you using?

[#3]

i suppose it is excel 2016

[#4]

@venkat1926 The .xlsx file type was introduced in Excel 2007, so unless you have changed the default settings (perhaps accidentally) your Excel version should always save in xlsx and be able to open it. So, I can't really help you any further.

[#5]

I did this change opened excel. clicked options. under general options clicked <save> there in "save files in this format. and selected "excel 97-2003 workbooks xls" and now any saving is done in this type.
reg. old xlsx files in pc I used the web "www.zamzar.com" and convert xlsx to xls. But some restrictions- only two files in a day.

[#6]

@venkat1926 

.xlsx files are much more powerful. I'd check your Windows associations and Excel trust centre to allows them.