This case has a number of important things to learn.
The PC that usually access DNS-323 crashed and I have to replace it. However, with the new PC, I’m not able to detect DNS-323 using using the Easy Search utility. Thinking that the config of DNS-323 is corrupted, I reset the unit with the reset button.
Still unable to find DNS-323 on Easy Search, so access it using IP address and got a shock. My personal files are gone. Only those that I share publicly are visible. Conncet the HDD to PC using SATA-USB and tried reading the disk using R-Studio but it looks the same as reading it thru the DNS-323.
Can anyone advise how can I recover my files
First of all, if the system was not affected by the failure, it does not need repairs. It was the PC that failed, not the NAS. The NAS was in all likelihood fine before reset. The trick to figure out what is the IP address of the NAS and then access it by IP should have worked without reset.
Finding the right IP is sometimes tricky. Obvious steps are to look at the router DHCP assignments table, which lists IP addresses given out by the router, if the network is using dynamic IPs, or you can go as far as a ping/port scan if addresses are static and you do not keep track of addresses. In case of static IPs, the best place to write down the IP for the NAS is on the NAS itself.
You can try Home NAS Recovery to see if it finds missing files (if the undelete is better than that in R-Studio, this might work), but if not, the data is lost. EXT filesystem does not go easy on deleted files.