i have downloaded a zip file and saved it in my desktop however i can't access it through the command line

Its a compressed file so you won't access it like that, that only works for binaries (programs). You need to run:

cd ~/Desktop
unzip opsys_proj1.zip

Be sure you match the case EXACTLY. Linux is VERY case sensitive

Actually this problem is not only with compressed file but also with normal files.So i fixed the problem of not finding the file but then there was another problem.It said that my access to the file was denied.What should i do??

is your username 'sotos'?

Yes...Could that be it???I would never think about it...Any suggestions??

I also have this problem....I try to unzip it with the unzip command and it still says that there is no file like that....What can i do about that???I have the opsys_proj1.zip file saved in my Downloads and this is what i get.The proj1 is just a directory i created.I want to unzip the file in this directory

sotos@ubuntu:~$ cd proj1
sotos@ubuntu:~/proj1$ unzip opsys_proj1.zip
unzip: cannot find or open opsys_proj1.zip, opsys_proj1.zip.zip or opsys_proj1.zip.ZIP.
sotos@ubuntu:~/proj1$

sudo chown $USER:$USER /home/sotos/Desktop/opsys_proj1.zip

will make your current user the owner of the file, run the command as given and it will work